1. Field of the Invention
The present invention relates to a method of effecting bidirectional communications by carrying out both an output operation to write output information transmitted from a master station into an input/output device and an input operation to transmit input operation from an input/output device to the master station. More particularly, the present invention is directed to such a bidirectional communication method which is suitable for effecting communications to control a number of input/output devices that are located remotely from each other, such as to remotely control robots, production lines, home-use appliances, display devices, etc., to control simple LANs (Local Area Networks), or to achieve fine control over temperatures and humidities using a number of temperature and humidity sensors that are positioned in remote locations.
2. Description of the Prior Art
Communication systems known as a LAN (Local Area Network) are widely used to effect communications between devices that are remotely spaced at distances of several meters or greater.
FIG. 11 of the accompanying drawings shows a frame of serial digital signals according to the data format of the LAN.
To exchange digital signals between a station (hereinafter referred to as a "master station") and another station (hereinafter referred to as a "slave station"), a frame of serial digital signals is generated by the master station and transmitted from the master station to the slave station.
As shown in FIG. 11, the frame usually includes a frame synchronizing signal at its leading end, which indicates the leading end of the frame. The frame also has, following the frame synchronizing signal, a control information field indicating the length of the frame, the type of the frame, etc., an address information field indicating a slave station designated to receive the frame, and an information field containing output information to be transmitted to the slave station. The frame also includes, following the output information field, an error detecting code such as a parity-bit code or a code produced using a check sum, for determining whether or not the output information contained in the output information field has been modified or rewritten by noise or the like during the transmission of the frame, and an end code indicating the trailing end of the frame. The end code may be omitted and the control and address information fields may be switched around in some frames.
The frame of the above structure is generated by the master station and transmitted to the slave station. The slave station which is designated by the address contained in the address information field receives the transmitted frame, and processes, e.g., stores, the output information according to the control information contained in the control information field. When information is to be sent from the slave station to the master station, the same procedure as the procedure described above is employed, and a frame similar to the above frame is generated by the slave station and transmitted to the master station.
The communication protocol (communication procedure or rules) described above allows a large amount of information efficiently from one station to another station.
There are instances where a command is to be transmitted from a master station to a slave station to energize a single lamp at the slave station, and the on/off information of a single pushbutton switch connected to the slave station is to be read by a master station. If the above communication protocol is employed in those cases which require communication of a relatively small amount of information, then the communication process will be very inefficient to carry out.
FIG. 12 of the accompanying drawings schematically illustrates a master station M and slave stations S.sub.1, S.sub.2 which communicate with each other according to the communication protocol as shown in FIG. 11. In FIG. 12, four input/output devices IO.sub.11, IO.sub.12, IO.sub.13, IO.sub.14 are connected to the slave station S1, and three input/output devices IO.sub.21, IO.sub.22, IO.sub.23 are connected to the slave station S.sub.2.
Now, it is assumed that the input/output device IO.sub.11 is a pushbutton switch, and the master station M is to recognize the on/off status of the pushbutton switch, i.e., whether the pushbutton switch is turned on or off.
The master station M generates a frame, as shown in FIG. 11, whose control and output information fields contain a written command to read the status of the push-button switch IO.sub.11 in the slave station S.sub.1 and transmit the read status to the master station M. The master station M then transmits the generated frame to the slave station S.sub.1. The slave station S.sub.1 then reads the contents of the received frame, and interprets the frame such that the slave station S.sub.1 is required to send the on/off status of the pushbutton switch IO.sub.11. The slave station S.sub.1 reads the status of the pushbutton switch IO.sub.11, newly generates a frame containing information as to the status of the push-button switch IO.sub.11, and transmits the generated frame to the master station M. The master station M receives the frame transmitted from the slave station S.sub.1, reads the contents of the frame, and recognizes the on/off status of the pushbutton switch IO.sub.11.
Both the master station M and the slave station S1 have respective microcomputers for generating frames and interpreting received frames. Therefore, each of the stations is complex in structure. Since it is necessary to design control communication software for each of the slave stations in addition to the master station, the period of time needed to design the necessary software is long, and the software thus designed is highly costly. The above many steps to be followed to transmit the information result in an inefficient, slow communication process for the exchange of a relatively small quantity of information.